U.S. and Japanese researchers last month reported they had reprogrammed human skin cells into behaving like embryonic stem cells, the body's master cells. They call the cells induced pluripotent stem cells, or iPS cells for short.

Hanna and colleagues working in Rudolf Jaenisch's lab at Whitehead Institute took skin cells from diseased mice and inserted four genes that reprogram the cells into becoming iPS cells.

Pluripotent or multipurpose cells, such as embryonic stem cells and the new cells, can morph into any type of cell in the human body.

The researchers then coaxed these mouse master cells into becoming blood-forming stem cells and substituted the faulty gene that causes sickle cell anemia with a working one.

When they transplanted these cells into the diseased mice, tests showed normal blood and kidney function, they report in Friday's issue of the journal Science.

The four genes needed to turn skin cells into master cells are delivered using a type of virus called a retrovirus.

"Once they enter the genome, there is the danger that they can silence some genes that are important or they can activate some dangerous genes that shouldn't be activated," Hanna said.

Another obstacle is that one of the four genes used is c-Myc, which is known to cause cancer.

Hanna and colleagues got around that by removing the c-Myc gene after it had done its job of converting the skin cells into iPS cells. "It is far from solving the problem," he said.

Scientists hope to use stem cells to treat a host of diseases like diabetes, Parkinson's disease and spinal injuries. And the new technique for making stem cells will make them easier to study.